1. Field of the Invention
The invention relates to a telecommunication line circuit including a Herter bridge with a first pair of terminals coupled to a bidirectional 2-wire line, a second pair of terminals coupled to an exchange and a third pair of terminals where longitudinal currents are eliminated from sensed signals, as well as a ringing generator that may be coupled across the second pair of terminals.
2. Description of the Prior Art
Such a line circuit is known from the U.S. Pat. No. 4,292,473 and makes use of the property of a Herter bridge to avoid the undesirable effects of longitudinal currents. As disclosed for instance in the U.S. Pat. Nos. 3,525,816 and 3,748,395, a Herter bridge (appearing in FIG. 2 of the present application) basically includes two high resistance potentiometers (R2+R3, R5+R4) cross-coupled between opposite terminals of two resistances (R0, R1) of low value, respectively interconnecting the first/second terminal of the first pair with the corresponding first/second terminal of the second pair, the tapping points of the potentiometers constituting the third pair of terminals. Thus, it is essentially a hexapole network with six resistances in a closed ring, (R0, R4, R5, R1, R3, and R2) and apart from being able to avoid the effects of longitudinal currents, it is also a bridge which can be balanced for a particular resistance value across the first pair of terminals (STB, SRA), i.e. the line loop. This means that an input signal fed to the second pair of terminals (STA, SRB), i.e. the exchange side, will not produce any output signal across the third pair of terminals (A, B), i.e. the detector, when such a particular or critical resistance is branched across the first pair of terminals.
This has been used to advantage in order to detect an open or closed line loop since by designing the elements of the Herter bridge so that it is balanced for a critical loop resistance value well below that of an open loop, i.e. a very high resistance, and well above that of a closed loop, i.e. a very low one, such changes of condition in the loop are readily detected by a change of polarity of the output signal at the third pair of terminals.
Calling v this output signal (between B and A), it can be expressed as the algebraic sum of the potential drops across R4, R0 and R2 in series as well as those across R5, R1 and R3 in series. This enables v to be expressed as a linear function of four currents (as shown in FIG. 2): i0 circulating through R0 towards the loop, i+i' from R0 into the loop, i-i', into R1 from the loop and i1 through R1 away from the loop. Evidently, i' represents the undesirable longitudinal current and since the i' component of v is found to be equal to half the potential drop across a resistance of value R4-R5+R2-R3, such longitudinal currents will be eliminated from v, the detected signal, provided EQU r=R2-R3=R5-R4 (1)
if the potentiometers are identical i.e. R2=R5 and R3=R4 so as to avoid i being involved in another relation including also i0, i'0 and i'. Then v is independent of i', i.e. EQU 2v=(r-R0)i0 +(r-R1)i1-2ri (2)
On the other hand, with r=0, i.e. R2 and R3=R4=R5, v is solely a linear function of i0 and i1.
When (1) is satisfied, the critical value R of the loop resistance for which the Herter bridge, immune longitudinal currents, is balanced, can be found by equating the potential drop across R (i'=0) to the algebraic sum of the potential drops across R4, R5 and R1 in series as well as to that across R0, R2 and R3 in series and for v equal to zero. This enables R to be expressed as ##EQU1##
In practice, the Herter bridge usually has not only a symmetrical topology but also symmetrical values since R0/R1, as well as R2/R5 and R3/R4 will be equal in pairs to secure a substantial longitudinal balance. This particular symmetrical case, entailing also R'0=R'1, leads to a much simpler expression for the critical value R, i.e. ##EQU2## It shows that, if only positive resistance values are considered for the various elements, the bridge can only be in a balanced condition when R2=R5 is larger than half the total resistance of each potentio-meter, the latter being usually chosen relatively high and even quite high in some applications, e.g. 240 kilo-ohms, in order not to waste energy in the detecting process, whereas R0=R1, i.e. the series feed resistances, are usually chosen quite low, e.g. 50 ohms, in order not to waste energy in feeding a substation through the line loop from the exchange.
For symmetrical values and with r=0, i.e. when R2=R3=R4=R5, (3') shows that a balance cannot be secured for positive values of both R and R'0 although in practice, with R0=R1 being much smaller than R2=R3=R4=R5, one has very nearly a simple balanced Wheatstone bridge involving these last four resistances when R is infinite, i.e. for an open loop. At the other end of the range, when R is zero, i.e. for a closed loop, the potentials at the two output terminals will respectively be of the order of 3/4 and 1/4 of the input voltage across the second pair of terminals.
The above considerations already indicate that various designs of the 6-resistance Herter bridge may all eliminate the longitudinal current component in the output, but additional variants involving a modified topology are still possible while retaining the general properties outlined, including the bridge effect. For instance, in the U.S. Pat. No. 3,622,709 one has added a seventh resistance linking one of the terminals of the third pair (B) to that terminal of the second pair (SRA) to which it is not connected by a resistance (such as R4 to STB) in the 6-resistance bridge.
These 6- and 7-resistance versions appear for instance in the French patent published under No. 2,324,181 and in the U.S. Pat. No. 4,103,112 respectively, and there the Herter bridge is used to detect an off-hook condition during ringing in a line circuit including a transformer with a split winding on the line side, the supervision signal at the output of the Herter bridge being unaffected by longitudinal currents.
Further line circuits using a Herter bridge in connection with ringing are also to be found in other patents such as the Belgian Pat. No. 846 034, British Pat. No. 1 511 767 as well as European patent application published under No. 0,096,473.
In the more recent types of telephone exchanges one resorts to the use of AC and DC loop impedance synthesis in order to effectively produce AC impedances and DC resistances of suitable characteristics and values from the two low-valued series feed resistances (R0, R1). Reference may for instance be made to U.S. Pat. Nos. 4,315,106, 4,317,963, and 4,387,273 as well as to European patent application published under No. 0,112,731. In this last application, a Herter bridge is used for dial pulse detection while in the embodiments of FIG. 8 of U.S. Pat. No. 4,317,963 and of FIG. 7 of U.S. Pat. No. 4,387,273, one has a Herter bridge with two cascaded grounded operational amplifiers in the feedback loop.